1) Field of the Invention
The present invention relates generally to an ion electric propulsion unit, and particularly to a microwave electron cyclotron resonance (ECR) propulsion unit in which the ECR plasma chamber is electrically biased to a positive potential.
2) Background of the Problem and Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Electric propulsion has long held promise over chemical rockets as a means to provide greater thrust via higher exhaust velocities. Electrothermal propulsion units electrically heat propellant that is then expanded through a nozzle to deliver thrust to the unit. Electrostatic propulsion units overcome fundamental thermal limitations (conversion of thermal energy into kinetic energy) of electrothermal units by accelerating propellant by application of an external body force. In a simple ion thruster, a beam of positive ions is accelerated by an electric field and subsequently neutralized by an equal flux of free electrons. In certain ion thrusters, discharge cathodes are used to produce the plasma which makes cathode life a key concern. Cathode failure can occur due to physical sputter erosion (from plasma on cathode ion bombardment in the plasma), and emitter element failure. Use of electron cyclotron resonance to generate the plasma eliminates the need for the cathode, potentially increasing the life of the thruster. Typically in ion thrusters, the ions are “extracted” from the plasma chamber and accelerated by electrode “grids” downstream of the plasma chamber. Use of such “grids” has certain disadvantages, including: a) “extracting” the ions is inefficient; b) electron backstreaming can occur causing the grid apertures to widen due to erosion; c) presence of conducting flakes from electrode erosion can lead to shorting that would terminate extraction; d) failure of the grid electrodes due to erosion; e) space charge limitations provide a fundamental limit on ion extraction. If too many ions collect near the grid, their “self field” will be strong enough to turn back other ions trying to pass the grid, limiting the ion current (“space charge limited current flow”).
U.S. Pat. No. 6,396,211 discloses a microwave discharge electrostatic accelerator propulsion device utilizing acceleration electrodes disposed in the discharge chamber.
Published U.S. Patent Application 2007/0234705 discloses an ECR thruster that utilizes magnetized ponderomotive force for accelerating the plasma.